A magnetic resonance imaging system is capable of irradiating a target placed in a static magnetic field with an RF magnetic field of a specific frequency to excite nuclear magnetization of each proton contained in the target (magnetic resonance phenomenon), detecting a magnetic resonance signal generated from the target and thereby acquiring physical/chemical information. Now-ubiquitous Magnetic Resonance Imaging (hereinafter abbreviated as MRI) acquires images on which a density distribution of protons principally contained in a molecule of water in the target has been reflected.
As opposed to MRI, there is known a method called Magnetic Resonance Spectroscopy (hereinafter abbreviated as MRS) which separates magnetic resonance signals every molecules with a clue as to the difference (hereinafter called chemical shift) between magnetic resonant frequencies due to the difference between chemical bonds of various molecules containing protons (refer to e.g., J. Granot, “Selected Volume Excitation Using Stimulated Echo (VEST). Applications to Spatially Localized Spectroscopy and Imaging”, J. Magn. Reson., vol. 70, pp. 488-492 (1986)).
A method for acquiring spectrums of a number of regions (pixels) simultaneously and performing imaging thereof every molecules is called Magnetic Resonance Spectroscopic Imaging (hereinafter abbreviated as MRSI). By using MRSI, a concentration distribution set every metabolites can visually be captured (refer to, for example, D. G. Norris, W. Dreher, “Fast Proton Spectroscopic Imaging Using the Sliced k-Space Method”, Magn. Reson. Med., vol. 30, pp. 641-645 (1993)).
When a living body is intended for measurement, the concentration of each metabolite is often very low. Therefore, when a signal is measured without suppression of water of high concentration upon execution of MRS or MRSI measurement, a weak signal of a metabolite is buried in a skirt or base of an enormous signal peak generated from water, so it is very difficult to separate/extract a metabolite signal. Therefore, the prior art performs processing for suppressing a water signal immediately before execution of excitation and detection in accordance with an MRS or MRSI measurement sequence (refer to, for example, D. G. Norris, W. Dreher, “Fast Proton Spectroscopic Imaging Using the Sliced k-Space Method”, Magn. Reson. Med., vol. 30. pp. 641-645 (1993)).
In the processing for suppressing the water signal, firstly, a transmission frequency is matched with a water peak position and an RF magnetic field in which an excitation frequency band width is narrowed to a water peak width or so is irradiated, in order to excite only nuclear magnetization contained in a water molecule. Next, the phases of nuclear magnetization contained in a number of water molecules each placed in an excitation state are rendered random and a dephase gradient magnetic field is applied to bring a vector sum of nuclear magnetization to zero (pseudo saturation). By performing excitation and detection in accordance with the MRS or MRSI measurement sequence while a pseudo saturation state of water magnetization is continuous, a weak metabolite signal was measured. Since the signal of each metabolite is very weak, a large number of averages are often performed to improve a signal-to-noise ratio (SNR) of each obtained spectrum in the conventional MRS or MRSI measurement.
As “a method for correcting a variation in magnetic resonant frequency with a change in static magnetic field strength” related to the present invention, there are known a report related to a method for performing the correction of a frequency variation in MRI (refer to, for example, Japanese Patent Application Laid-Open No. 2002-291718) and a report related to a method for performing the correction of a frequency variation in MRSI (refer to, for example, Japanese Patent Application Laid-Open No. Sho 63-230156).